Fast GPU Histogram Analysis for Scene Post-Processing

Andy LuedkeHalo Development TeamMicrosoft Game Studios

Why do Histogram Analysis?

» Dynamically adjust post-processing settings based on rendered scene content

» Drive tone adjustments by discovering intensity levels and adjusting tonemapper settings

» Make environments feel consistent with a wide range of illumination

» Mimic eye’s natural adaptation to exposure and focal ranges

Existing Techniques

» Average Scene Luminance Varies significantly with small

perceived changes in HDR scenes

» Luminance Histogram Provides more useful exposure data Limited by fixed number of bins CPU generated from locked texture

Adjustable granularity, poor performance GPU queries to update histogram bins

Low granularity, delayed scene response

Luminance Histogram

» Used to find interesting exposure control points Median luminance (50th percentile) Bright point (90th – 95th percentile)

» Search histogram for each point» Only contains luminance data from

previously rendered frames» Expensive to generate and search» Histograms are not great for

exposure control

Sorted Luminance Buffer

» Sorting the luminance fixes many problems with histogram method

» Expensive to sort on the CPU» Sort on the GPU instead

Parallel sorts are quite fast on GPUs Works on current frame’s data

» Easy to find percentiles in a sorted luminance buffer Sample center of buffer for median

value, or at X*N/100 for Xth percentile

GPU Sorting

» Avoids histogram range clamping and bin granularity problems

» Works on current frame’s values» Sorts multiple channels at once

Sort luminance and depth in a two channel buffer, or more in 4 channels

» Sorted buffer remains on GPU CPU processing of exposure control

can be moved to the GPU exclusively

GPU Sorting (continued)

» Bitonic sort works well on the GPU Well suited for shader implementation Exactly ½*(log2n * (log2n+1)) passes

» Scale to slower hardware by reducing size of sorting buffer Exposure control point lookups are still

direct, but have less resolution

» Bitonic sort works best on power of 2 textures, but can be tweaked to work on other sizes

Bitonic Sort Demo» Red = Average luminance» Green = Maximum luminance

GPU Exposure Processing

» Shader samples sorted luminance buffer and outputs updated exposure control values Use GPU to sample many points and

do complex adjustments (curves, etc) Blend new exposure control values

with previous values over time

» Another shader generates further tonemapping settings Bloom settings, saturation, tone, etc.

Local Exposure Control

» Use one channel of sort buffer as a key for another channel’s sort Sort regions of the screen in addition

to the full frame’s values Still direct access as long as each

region has a known number of pixels RGBA=[Lum, Depth, Local Lum, Key]

» Allows you to divide the screen into multiple exposure zones and mix local and global adjustments

Keyed Luminance Sort» Red/Green = Avg/Max Luminance» Blue = Regional Avg Luminance

Local Exposure Control

» Use different region masks to customize to your game’s needs

» Must know how many pixels in each region for direct value access

Focal Range Control

» Sorted depth gives useful information for DOF ranges

» Detect changes in depth range, adjusting DOF settings to simulate eye’s adjustment of focal range

» Extracting depth has sampling cost, but no additional sorting cost May not need to filter on downsample

CPU Exposure Pipeline

GPU Exposure Pipeline

Questions?» aluedke@microsoft.com» Please fill out your surveys

References» GPU Gems, Chapter 37» GPU Gems 2, Chapter 46» UberFlow: A GPU-based particle

engine [Kipfer, et al.]» Wikipedia, Sorting Algoritms